Overcoming the iPhone's storage limitations to some extent, Box.net has introduced an optimized version of Box.net for the device. The service allows users to upload any file to its servers then remotely access them. Of course, the file format must be supported on the iPhone, so right now only Word, Excel, PDF documents, iPhone-encoded movies and images will be accessible.
The basic, free account includes 1GB of storage and has a 10MB-per-file limit. A 5GB account is $7.95 per month. The upgraded account has a 1GB per-file limit.
Of course, you could always email file attachments to a free Gmail account and access them that way, but Box.net's interface is much more intuitive for browsing, organizing and accessing documents.
The iPhone-optimized version of Box.net is available at http://i.box.net.